One end of such devices is brought into contact with a molten mass of metal contained in the furnace, for example a bath of steel, whilst its other end is connected to a terminal of an electrical direct-voltage supply. The other terminal of the supply is conventionally connected to one or more movable vault electrodes usually located at a short distance above the bath of metal, to generate and maintain between them and the bath surface electric arcs which provide the thermal energy required for melting and processing the metal.
Such electrical connection devices are known in iron and steel industry as a hearth electrode, especially in processing or temperature-maintaining ladles or in arc furnaces for the smelting of solid metal-bearing materials.
One of the essential problems encountered in this field relates to the long-term resistance of these hearth electrodes which are subjected to severe thermal stresses because of their contact with the bath of molten metal and the high intensities of the electrical current passing through them, especially as regards arc furnaces, where these intensities can reach values of the order of 40,000 A and even above.
The applicant, who has given these problems much study, proposed, in French Patent Application No. 84/17323 of 6th Nov. 1984, a hearth electrode for a direct-current arc furnace which is particularly well suited to very long operating periods. This hearth electrode comprises essentially a steel core which passes through the wall of the furnace for coming with one end into contact with the molten mass of metal present in the furnace, the other end of which being encased in a sleeve made of thermally and electrically conductive material, advantageously copper. This sleeve, cooled by circulation of water, surrounds initially the core at a distance, so as to form a separation space between them. The latter is cyclically filled during "hot" periods, that is to say when the furnace is put into operation, and then reformed during the "cold" periods when the core cools after the current has been cut off. This contributes to allowing the steel core to expand and contract longitudinally within the sleeve in time with the repetitive remelting/resolidification sequences to which its portion not surrounded by the cooled sleeve is subjected and which result from the cyclical operation of the furnace.
Furthermore, the steel core is provided, at its end contained in the cooled sleeve, with a plug made of thermally and electrically conductive material which, like the sleeve, advantageously consists of copper cooled as a result of the internal circulation of a cooling fluid, and which is connected to a terminal of an electrical supply.
A hearth electrode of this type produces excellent results. Nevertheless, it was possible to observe by means of a displacement sensor that, during the furnace shutdown periods (or "cold" periods), the core and the attached plug therewith behaved in the manner of a sliding part which rose in the sleeve, and that this rise was repeated sequentially in the course of the remelting/resolidification cycles and could result in the gradual disappearance of the hearth electrode because of the consumption of its end in contact with the bath of metal.